© CAMBRIDGE UNIVERSITY PRESS 1983, 1993

1.7 Coordinates and Catalogues of Astronomical Objects

Before proceeding further we will describe how the astronomer locates the position of a heavenly body in the sky. In general the astronomer does not know the distance of the body from us; he sees it projected on the sky, on what is known as the celestial sphere. Two coordinates, akin to longitude and latitude, are therefore needed to specify the position of the body on the sphere.

Figure 1.20 shows two different coordinate systems, both useful to the astronomer in different contexts. The system in Figure 1.20(a) uses right ascension (RA, denoted by ) and declination (), coordinates fixed by the geometry of the Sun-Earth system. Here the poles are the points N, S on the celestial sphere where the Earth's axis of rotation intersects it. The celestial equator is the great circle on the celestial sphere whose plane is perpendicular to NS. The plane in which the Sun appears to go round (as seen from the Earth) intersects the celestial sphere in another great circle called the ecliptic. The ecliptic and the celestial equator intersect in two points and , corresponding to the position of the Sun on 21 March and 22 September, respectively. Now and are the longitude and latitude of a celestial object measured with respect to the celestial equator and the great circle through N, , S, and . This latter circle, known as the celestial meridian, plays the role of the Greenwich meridian on the Earth, with the point of zero . It is customary to measure in hours and minutes, with the range 360° corresponding to 24 hours. The declination is written in degrees, minutes, and seconds, with + for North, - for South.

 Fig. 1.20. This figure demonstrates how to measure (, ) and (l, b) for an object Q in the sky using two different coordinate systems. (a) The coordinate system based on the geometry of the Sun-Earth system. (b) The coordinate system based on the geometry of our Galaxy.

 Name Type of object Catalogue code Messier Nebulae and galaxies M followed by catalogue number. New General Nebulae and galaxies NGC followed by catalogue number in galaxies increasing RA. Abell Clusters A followed by catalogue number in increasing RA. Cambridge (3rd, 4th, 5th surveys) Radio sources 3C, 4C, SC followed by catalogue number in increasing order of RA. Ohio source Radio sources O followed by a letter (B to Z omitting O and a number. The letter gives hours of RA, the first digit the declination in 10° intervals, and the last two digits the decimal part of the RA to two places. Thus 1443 + 101 is OQ 172.

While (, ) coordinates are convenient for measurements made from the Earth, the cosmologist is often interested in knowing how the object is located vis-à-vis the plane of the Galaxy. For such purposes the galactic coordinates are useful. These are illustrated in Figure 1.20(b). The galactic equator is the great circle where the plane of the Galaxy intersects the celestial sphere. N, S are the North and South galactic poles, while the ``zero'' meridian is the one passing through the points N, S, and the point C where the direction from Earth to the centre of the Galaxy meets the celestial sphere. This meridian is also called the galactic meridian. The galactic longitude is denoted by l, and latitude by b. In terms of the (, ) system, the point C has the coordinates 17h42m.4, -28°55'. It is possible to convert from one coordinate system to another using spherical trigonometry.

Astronomical objects are catalogued in many ways. Table 1.2 lists some of the catalogues referred to in this book and their code letters. This is not an exhaustive list, but is given as an illustration of how sources are numbered and listed. A more systematic method common in recent compilations is to list the object by its (, ) values in the form (±).

Thus the object 1143-245 has right ascension 11h43m and declination -24°30' ( - 24.5°).